The invention relates to granular polycrystalline silicon and to the production thereof.
Granular polycrystalline silicon or granular polysilicon for short is an alternative to the polysilicon produced in the Siemens process. While the polysilicon in the Siemens process is obtained as a cylindrical silicon rod which has to be comminuted prior to further use thereof in a time-consuming and costly manner to give what is called chip poly, and may again have to be purified, granular polysilicon has bulk material properties and can be used directly as a raw material, for example for single crystal production for the photovoltaics and electronics industry.
Granular polysilicon is produced in a fluidized bed reactor. This is accomplished by fluidization of silicon particles by means of a gas flow in a fluidized bed, the latter being heated to high temperatures by means of a heating apparatus. Addition of a silicon-containing reaction gas results in a pyrolysis reaction at the hot particle surface. This deposits elemental silicon on the silicon particles, and the individual particles grow in diameter. The regular removal of particles which have grown and addition of relatively small silicon particles as the seed particles (called “seed” later in the document) enables continuous operation of the process with all the associated advantages. The silicon-containing reactant gases described are silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4) and mixtures of these gases with hydrogen. Such deposition processes and apparatuses for this purpose are known, for example, from U.S. Pat. No. 4,786,477.
The granular silicon obtained from the deposition processes features high purity, i.e. a low content of dopants (especially boron and phosphorus), carbon and metals.
U.S. Pat. No. 4,883,687 discloses granular silicon defined in terms of the particle size distribution, the boron, phosphorus and carbon contents, the surface dust content, and the density and bulk density thereof.
U.S. Pat. No. 4,851,297 describes doped granular polysilicon, and U.S. Pat. No. 5,242,671 granular polysilicon with a reduced hydrogen content.
U.S. Pat. No. 5,077,028 describes a process in which granular polysilicon which features a low chlorine content is deposited from a chlorosilane.
The granular polysilicon nowadays produced on a large scale has a porous structure, and two seriously disadvantageous properties resulting from this:
Gas is enclosed in the pores. This gas is released in the course of melting and disrupts the further processing of the granular polysilicon. Attempts are therefore made to reduce the gas content of the granular polysilicon. However, as described in U.S. Pat. No. 5,242,671, an additional working step is needed, which increases the production costs and additionally causes additional contamination of the granules.
The granular polysilicon is not particularly abrasion-resistant. This means that the handling of the granules, for example in the course of transport to the user, gives rise to fine silicon dust. This dust is disruptive in several ways:
it is disruptive in the further processing of the granular polysilicon since it floats as the granules are melted;
it is disruptive in the course of transport of the granular polysilicon within the production plant because it causes deposit formation on pipelines and results in blockage of valves;
it is a potential contamination carrier owing to its high specific surface area.
Abrasion already leads to losses in the fluidized bed in the course of production of the granular polysilicon.
Disadvantageously, in the course of production based on monosilane as the silicon-containing reactant gas, amorphous silicon dust is formed directly as a consequence of a homogeneous gas phase reaction, in addition to the abrasion in the deposition process.
This ultrafine dust can partly be removed from the product, but this likewise means inconvenience, material loss and hence additional costs.
U.S. Pat. No. 7,708,828 discloses granular polycrystalline silicon consisting of particles having a density greater than 99.9% of the theoretical solid density and hence a pore fraction less than 0.1% and a surface roughness Ra less than 150 nm. The particles preferably have a dopant content of boron less than 300 ppta, preferably less than 100 ppta. The particles preferably have a carbon content less than 250 ppba, preferably less than 100 ppba. The particles preferably have a total content of the metals Fe, Cr, Ni, Cu, Ti, Zn and Na of less than 50 ppbw, preferably less than 10 ppbw.
The inventive granular polysilicon can preferably be produced in a radiation-heated fluidized bed reactor.
The inventive high-purity granular polycrystalline silicon is preferably produced by deposition of a reaction gas on seed crystals of silicon in a fluidized bed. The reaction gas consists preferably of a mixture of hydrogen and silicon-containing gas, preferably halosilanes, more preferably of a mixture of hydrogen and trichlorosilane (TCS). The deposition is preferably effected at a temperature of the fluidized bed within the reaction range from 700° C. to 1200° C. The initially charged seed crystals in the fluidized bed are fluidized with the aid of a silicon-free fluidizing gas, preferably hydrogen, and heated by means of thermal radiation. The heat energy is introduced homogeneously over the area of the fluidized bed by means of flat radiant heaters. In the reaction zone, the silicon-containing reaction gas is deposited on the silicon particles as elemental silicon owing to a CVD reaction. Unreacted reaction gas, fluidization gas and gaseous reaction by-products are removed from the reactor. By regularly drawing off particles provided with the deposited silicon from the fluidized bed and adding seed crystals, the process can be operated continuously.
The temperature of the fluidized bed in the reaction region is preferably from 850° C. to 1100° C., more preferably from 900° C. to 1050° C., most preferably from 900° C. to 970° C.
The reaction gas can be injected into the fluidized bed via one or more nozzles.
The concentration of the silicon-containing reaction gas, based on the total gas rate through the fluidized bed, is preferably 10 mol % to 50 mol %, more preferably 15 mol % to 40 mol %. The concentration of the silicon-containing reaction gas in the reaction gas nozzle is, based on the total gas rate through the reaction gas nozzles, preferably from 20 mol % to 65 mol %, more preferably from 30 mol % to 65 mol %, most preferably 40 mol % to 60 mol %.
In the course of production of the granular polysilicon, only slight dust formation occurs. This and the lower level of abrasion lead to increased yields, since barely any fine dust is discharged from the fluidized bed, this always leading to a material loss in known processes.
However, the granular polysilicon known from the prior art does not have particularly good pulling properties (melt properties, lead frequencies, lead times). A description of the problem of lead frequency and lead time can be found, for example, in the Wacker Siltronic AG patent DE 10025870A1, paragraphs 0004, 0016 and 0018, and in DE19847695A1.
Part of the solution to the problem is the production of granules with much greater granule particle sizes. Production processes for large granule particle sizes require very high gas rates for fluidization in the fluidized bed reactor. In fluidized beds without a fixed bed zone in the bottom region, the greater movement of solids apparently results in contamination through the bottom region.
For the production of high-purity granular polysilicon, high-purity seed crystals are required.
Air jet mills which grind with high purity—as described in U.S. Pat. No. 7,490,785—cannot be used for large particle sizes with a mass-based median value of greater than 1250 μm.
The technical solution to date has been the use of a roll crusher. According to the abstract to JP 57-067019 (Shin Etsu Hondatai), silicon seed particles are obtained from granular silicon by comminuting them in a double roll crusher and then fractionating them by a screening operation.
Contamination of the silicon seed particles with other elements is prevented by virtue of the surface of the rolls being provided with a layer of silicon. The silicon-silicon material pair between roll and milling material, however, leads to high wear of the silicon layer on the rolls, such that only short service lives of the machine are possible before the rolls have to be exchanged. Economically viable production of seed is thus impossible.
A significant improvement with regard to roll wear is provided by the use of rolls with a hard metal surface and matched roll gap geometry, as described in DE 102004048948, but this results in contamination of the seed with B, C, Zn, Ti, Mg, W, Fe, Co, Sb and Zr.
The use of a seed mixture of screen undersize and seed ground with roll crushers ensures reduced contamination, but this is always still too high for processes such as GFZ pulling or for crucible pulling processes in the semiconductor industry. The GFZ (granular float zone) process provides for the production of a single crystal of silicon using molten granules. The process and apparatuses suitable for performance thereof are described, for example, in DE102010006724A1.
The problems described gave rise to the objective of the invention.
The object is achieved by the invention described herein.